Acceleration sensor element, acceleration sensor, and method of manufacturing the same

ABSTRACT

An acceleration sensor element including a weight, a support having a hollow part which is disposed around the weight as a center, and a flexible plate having piezoelectric elements including a piezoelectric body sandwiched by at least one set of electrodes and disposed horizontally across the support to suspend the weight at the hollow part of the support. Acceleration applied from the prior is converted into deflection of the flexible plate based on movement of the weight corresponding to the acceleration, and the direction and rate of the acceleration are three-dimensionally detected by electric charges generated in the piezoelectric body corresponding to deflection of the flexible plate. The piezoelectric elements are disposed only at flexible parts of the flexible plate.

TECHNICAL FIELD

The present invention relates to an acceleration sensor element for three-dimensionally determining the direction and the rate of acceleration applied from the exterior using a piezoelectric body, to an acceleration sensor therewith, and to the manufacture thereof.

BACKGROUND OF THE INVENTION

In the automobile industry and the machinery Industry demands for sensors capable of accurately determining physical quantities such as force, acceleration, and magnetic force are increasing. In particular, small sensors capable of detecting physical quantities for each component in two-dimensions or three-dimensions are required.

For example, a sensor in which a plurality of piezoelectric bodies are mounted on a flexible plate having an operation body is disclosed in Japanese Unexamined Patent Application Publication No. 5-26744.

The sensor is constructed so that the flexible plate may be deflected corresponding to physical quantities applied to the operation body from the exterior, and can three-dimensionally determine the direction and the rate of the physical quantities by electric charges generated in the piezoelectric body depending on the deflection of the flexible plate using a single sensor element.

The sensor element will be described using as an example, an acceleration sensor having a weight as the operation body. As shown in FIG. 2, when acceleration a is applied to the sensor element from the exterior, an inertial force f is applied to a weight 10 in a direction opposite to the acceleration a, and as a result, deflection 14 due to the inertial force f is generated at a flexible plate 12 horizontally disposed between the weight 10 and a support 11. Since electric charges depending on the direction and the rate of the deflection 14 are generated at a piezoelectric body 13 constructed over the flexible plate 12, the acceleration applied from the exterior can be three-dimensionally determined by measuring the electric charges.

In the sensor element as shown in FIGS. 3(a) and 3(b), the center of a base of the cylindrical weight 10, on which the flexible plate 12 is horizontally disposed, is specified as the origin O, a surface including the origin O, which is parallel to the flexible plate 12, is specified as an X-Y plane, the X-axis and Y-axis are specified so as to perpendicularly intersect each other in the X-Y plane, and the Z-axis is specified to Include the origin O and to perpendicularly intersect the X-Y plane.

In this instance, when it is assumed that a part sandwiched by one set of an upper electrode and a lower electrode of The piezoelectric body 13 is “one piezoelectric element,” for example, four elements of piezoelectric bodies and electrodes, each corresponding to the X-axis and the Y-axis, and eight elements of piezoelectric bodies and electrodes corresponding to the Z-axis can be arranged on the flexible plate 12.

In this case, the magnitude of each inertial force f applied to the weight 1 by the acceleration a applied from the exterior is determined as follows: the magnitude of an inertial force f_(X) in the X-axis direction as shown in FIG. 4(a) is determined by the amounts of the electric charges generated at “piezoelectric elements” E₁ to E₄; the magnitude of an inertial force f_(Y) in the Y-axis direction is determined by the amounts at E₅ to E₈ (not shown); and the magnitude of an inertial force f_(Z) in the Z-axis direction as shown in FIG. 4(b) is determined by the amounts at “piezoelectric elements” E₉ to E₁₂ and E₁₃ to E₁₆ (not shown).

In addition, the directions of the inertial forces are each determined from the polarity patterns of the electric charges (e.g., for a piezoelectric body top surface in FIG. 4(a), [+−+−] from the left; and for a piezoelectric body top surface in FIG. 4(b), [+−−+] from the left).

The inertial force f as a resultant force of f_(X), f_(Y), and f_(Z) determined as described above, and the direction and the rate of the acceleration a applied from the exterior can be three-dimensionally determined by a single small sensor element.

In the aforesaid sensor element, the entire flexible plate does not deflect uniformly; instead, the deflection stress rises rapidly, particularly at a boundary portion of the weight or the support and the flexible plate. Accordingly, when the piezoelectric elements are disposed at the boundary portion, the electric charges generated at the piezoelectric elements rapidly increase or decrease depending on slight deviations in the arrangement positions of the piezoelectric elements or variations in the shape of the weight or the support, so that it is possible to cause not only a variation of the output from each axis but also degradation in the sensor accuracy.

In addition, when the piezoelectric elements are disposed at a boundary portion, it is possible to cause degradation in sensor reliability due to the following deficiencies.

First, when the sensor element receives a rapid acceleration, a larger bending stress is applied to the boundary portion compared to other parts, and as a result, a particularly large force will be exerted not only on the piezoelectric elements but also on the piezoelectric body at the boundary portion Since the piezoelectric body Is made of piezoelectric ceramics such as PZT and PMN, whose mechanical characteristics such as strength and toughness are generally inferior, there is a concern that the piezoelectric body may be damaged. Second, since the behaviors of the pliable flexible plate and the rigid support or weight when the impact is applied are different, there is a concern that the piezoelectric elements may peel off from the flexible plate even if the piezoelectric body Is not damaged with the construction in which lower surfaces of the piezoelectric elements contact both the flexible plate and the support or the weight.

In addition, in the aforesaid sensor element, the flexible plate is required to have high flexibility in order to have sufficient sensitivity, while the weight and support are required to have high rigidity and to be unlikely to deflect in order to accurately detect the acceleration applied thereto.

If the sensor element is constructed by assembling the members such as the weight, the support, and the flexible plate, which were manufactured separately, the aforesaid opposed characteristics can be satisfied; however, there are problems in that many components and steps are required, resulting in lowered productivity.

As means for reducing the number of components and steps and for improving productivity, integral forming by the same material can be possible. When integral forming by ceramics is used as an example, the following methods may be mentioned: a ago method of cutting a ceramic green body 33 formed by filling a mold 30 with a ceramic power 31, and by conducting uniaxial pressing, etc., as shown in FIG. 5; a method of filling a mold 30 with a ceramic powder 31 and forming for a sensor 34, and then performing uniaxial pressing as shown in FIG. 6; a method of injection molding ceramic slurry 32 as shown in FIG. 7; and a method of molding by slip casting.

In any of the aforesaid methods, however, since the strength of the flexible part of the integral compact is extremely low and the dispersion of the density distribution is large, it is difficult to manufacture the flexible plate thinly and to precisely control the thickness of the flexible plate. That is, in the sensor element formed by the above method, the flexible plate is unlikely to deflect and the degree of deflection differs for each flexible plate or depending on a portion thereof.

Accordingly, in addition to the drawback of low sensitivity, there is a case in which the sensitivity differs for each sensor element, even if the same acceleration is applied, and also when the sensitivity differs for each axis, the direction and the rate of the acceleration obtained from the resultant force thereof are inaccurate, so that sensor accuracy is degraded.

The present invention is made in consideration of the above problems, and the objects thereof are to further improve sensor sensitivity and sensor accuracy while offering the advantages of a sensor element that is small and that can measure three-dimensional physical quantities using a single sensor element, and to provide a manufacturing method in which the sensor element can be easily manufactured.

SUMMARY OF THE INVENTION

The present invention provides an acceleration sensor element (hereinafter referred to as a “sensor element”) including a weight, a support having a hollow part, which is disposed around the weight as a center, and a flexible plate having piezoelectric elements in which a piezoelectric body is sandwiched by at least one set of electrodes. The flexible plate is disposed horizontally across the support so as to suspend the weight at the center of the hollow part of the support. Acceleration applied from the exterior is converted to a deflection of the flexible plate based on the behavior of the weight corresponding to the acceleration, and the direction and the rate of the acceleration are three-dimensionally determined by electric charges generated from the piezoelectric body corresponding to the deflection of the flexible plate. The piezoelectric body is disposed only at flexible parts of the flexible plate.

The sensor element according to the present invention is preferably a sensor element having the support the weight, and the flexible plate integrally sintered by a green sheet lamination technique. In addition, it is preferably an acceleration sensor element having the support, the weight, and the flexible plate integrally formed, in which the crystal structure or the crystal-grain diameter of at least one of the support, the weight, and the flexible plate differs from that of the others.

Furthermore, the sensor element according to the present invention is preferably a sensor element having a hole penetrating perpendicularly in the center of the weight so that the space above the flexible plate and the support hollow part may communicate with each other, in which narrow grooves communicating with at least the space at the side of a support outer periphery are formed at the positions on a support lower-end surface, which are each symmetrical with respect to the straight lines passing through the support center on the plane including the support lower-end surface, and in which the weight is suspended so that the plane including the support lower-end surface will not contact the lower-end surface thereof.

In addition, the present invention provides a manufacturing method for the acceleration sensor element, including a first step of laminating and compression bonding a green sheet of the configuration (in cross section) of the weight, the support and the flexible plate to form a laminated layer, a second step of integrally sintering the laminated layer to form a sintered body, and a third step of forming the piezoelectric elements on the sintered body by a thick-film method, and then sintering them.

In the manufacturing method according to the present invention, through holes are preferably punched in the center of the green sheet of the configuration in cross section of the weight and the flexible plate and at least two corners of the green sheet of the configuration in cross section of the support and the flexible plate, and laminating and compression-bonding are preferably conducted to make the laminated layer in a state where the through holes are penetrated by support pins.

In addition, in the manufacturing method according to the present invention, it is preferable that after the green sheet of the configuration in cross section of the weight, the support, and the flexible plate has been laminated and compression bonded on a planar green sheet to form a laminated layer, and then the laminated layer has been integrally sintered to form a sintered body, the sintered planar green sheet is cut in order that the lower end of the weight can be moved.

Furthermore, in the manufacturing method according to the present invention, it is preferable that after the planar green sheet has been placed on a green sheet of the configuration in cross section of the support, and furthermore the green sheet of the configuration in cross section of the weight, the support, and the flexible plate has been laminated and compression bonded to form a laminated layer, and then the laminated layer has been integrally sintered to form a sintered body, the sintered planar green sheet is cut in order that the lower end of the weight can be moved.

Furthermore, the present invention provides an acceleration sensor manufacturing method characterized in that the acceleration sensor element according to the present invention is fixed on a circuit board and is sealed by a cover in a dry atmosphere with a dew point of −40° C. or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a top view showing an embodiment of a sensor element according to the present invention and FIG. 1(b) is a front sectional view showing an operation state thereof.

FIG. 2 is a schematic perspective view showing an embodiment of the sensor element used in the present invention.

FIG. 3(a) is a schematic perspective view showing an embodiment of the sensor element used in the present invention, and FIG. 3(b) is a top view thereof.

FIGS. 4(a) and 4(b) are schematic explanatory views showing an operation state of the sensor element used in the present invention.

FIG. 5 is a schematic explanatory view showing an example of conventional integral ceramic molding.

FIG. 6 is a schematic explanatory view showing an example of conventional integral ceramic molding.

FIG. 7 is a schematic explanatory view showing an example of conventional integral ceramic molding.

FIG. 8(a) is a schematic perspective view showing an embodiment of the sensor element used in the present invention and FIG. 8(b) is a top view thereof.

FIG. 9(a) is a top view showing an embodiment of a conventional sensor element and FIG. 9(b) is a front sectional view showing an operation state thereof.

FIG. 10 is a schematic view showing the effect of the conventional sensor element when piezoelectric elements are deviated.

FIG. 11(a) is a top view showing an embodiment of the sensor element according to the present invention and FIG. 11(b) is a front sectional view thereof.

FIG. 12(a) is a bottom view showing an embodiment of the conventional sensor element and FIG. 12(b) is a bottom view showing an embodiment of the sensor element according to the present invention.

FIG. 13(a) is a top view showing an embodiment of the sensor element according to the present invention and FIG. 13(b) is a front sectional view thereof.

FIG. 14 is a process view showing an example of a laminated-layer manufacturing method.

FIG. 15 is a process view; showing an example of a laminated-layer manufacturing method.

FIG. 16 is a process view showing an example of a laminated-layer manufacturing method.

FIG. 17 is a process view showing an example of a laminated-layer manufacturing method.

FIG. 18 is a process view showing a piezoelectric-element forming method.

FIG. 19 is a process view showing an example of a cutting method of a planar green sheet.

FIG. 20(a) is a top view and a front sectional view showing an embodiment of the conventional sensor element and FIG. 20(b) is a top view and a front sectional view showing an embodiment of the sensor element according to the present invention.

FIG. 21 is a process view showing a manufacturing method of an acceleration sensor according to the present invention.

FIG. 22 is a process view showing a manufacturing method of the acceleration sensor according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail hereinbelow with reference to the drawings.

A sensor element used in the present invention includes, as shown in FIGS. 3(a) and 3(b) for example, a weight 10, a support 11 disposed around the weight 10 as a center, and a flexible plate 12 having piezoelectric elements in which a piezoelectric body 13 is sandwiched by at least one set of electrodes, which is disposed horizontally between the facing supports 11 while suspending the weight 10.

In the sensor element, the flexible plate 12 is deflected by the force exerted on the weight 10 based on the rate and the direction of the acceleration applied from the exterior, and the rate and the direction of the acceleration are determined in three-dimensions by the amount and the electrode patterns of electric charges of the piezoelectric elements generated corresponding to the deflection.

According to the present invention, the support 11 is a member for supporting the flexible plate 12 and the weight 10 disposed around the weight 10 as a center.

Accordingly, although the shape, material, etc., of the support are not limited so long as it has a strength capable of supporting the flexible plate 12 and the weight 10, the support preferably has an external shape of a rectangular prism and has a cylindrical hollow part 16 as shown in FIGS. 3(a) and 3(b), and is preferably constructed of a ceramic that has high rigidity and which is unlikely to be influenced by electromagnetic waves so that processing is facilitated and the symmetry with respect to acceleration in the X-Y plane is relatively high.

According to the present invention, the weight 10 is a member suspended in a manner such that the upper end surface thereof comes into contact with the flexible plate 12 to produce deflection at the flexible plate 12 by the force generated based on the rate and the direction of acceleration applied from the exterior, and which may be adhered to the flexible plate 12 using an adhesive agent, etc., or which may be integrally formed with the flexible plate 12.

The shape of the weight is not particularly limited so long as the weight can be suspended so that the upper end surface thereof may come into contact with the flexible plate, and a cylindrical shape or a shape in which the exterior diameter continuously or discontinuously changes such as a frustum of a cone and a different-diameter cylinder is possible. However, a cylindrical shape as shown in FIGS. 3(a) and 3(b) is preferable from the viewpoint of a high degree of symmetry with respect to the acceleration in the X-Y plane.

Although the material is also not particularly limited, the weight is preferably constructed of a ceramic, which is a material having a sufficient mass and a high density, which is unlikely to be influenced by electromagnetic waves, and which has a low thermal expansion coefficient.

According to the present invention, the flexible plate 12 is a flat member having the piezoelectric elements, which is disposed horizontally between the facing supports 11 while suspending the weight 10, and the entire flexible plate may be constructed of a piezoelectric body.

Although the shape and material, etc., of the flexible plate is not particularly limited so long as it has flexibility and is not damaged by the movement of the weight, the material is preferably a ceramic and has a high modulus of direct elasticity, and which is likely to induce distortion in the piezoelectric body, and as for the shape, it is preferable that it be constructed as a single flat body as shown in FIGS. 8(a) and 8(b) because it can be easily processed.

The flexible plate has the piezoelectric elements in which the piezoelectric body 13 is sandwiched by at least one set of electrodes. Although piezoelectric ceramics such as PZT, PMN, and PNN, or an organic piezoelectric body can be used as the piezoelectric body, it is preferable to use PZT having superior piezoelectric characteristics.

As for the piezoelectric body, although the flexible plate itself may be constructed of the piezoelectric body, the piezoelectric body 13 may be disposed over the entire flexible plate 12 as shown in FIGS. 9(a) and 9(b), and also the piezoelectric body 13 may be radially disposed on the disk-shaped flexible plate 12 while setting the center of the weight 10 as the origin O as shown in FIGS. 8(a) and 8(b).

According to the present invention, the part in which the piezoelectric body is sandwiched by a set of electrodes (an upper electrode and a lower electrode) from the upper and lower surfaces is called a “piezoelectric element”. Accordingly, the number of the elements can be increased depending not only on the number of the piezoelectric bodies, but also by the way in which the electrodes are arranged.

The deflection of the flexible plate generated based on the direction and the rate of acceleration applied from the exterior can be electrically determined by the piezoelectric elements.

The sensor element according to the present invention is characterized In that the aforesaid sensor element has the piezoelectric elements disposed only at low-rigidity, compared with the support or the weight, flexible parts of the flexible plate. According to the construction, residual stress in the sensor element based on differences in thermal expansion among the piezoelectric bodies, the electrodes, and the flexible plate in the sintering shrinkage or cooling process of the piezoelectric body is absorbed by the flexible plate, so that the relative position with respect to the weight and the support does not change, thereby yielding a highly accurate sensor element.

The sensor element as in FIGS. 1(a) and 1(b) (hereinafter referred to as “sensor element A”) is an example in which the aforesaid structure is applied to the sensor element. Assuming that part of the disk-shaped flexible plate 32 disposed horizontally between the weight 30 and the support 31, which are fixed to upper end surfaces of the weight 30 and the support 31 are nonflexible parts 32 a, and portions other than that are absolute flexible parts 32 b, the piezoelectric elements 34 are disposed only at the flexible parts 32 b.

When an acceleration a is applied to the sensor element A from the exterior, an inertial force f is applied to the weight 30 in the opposite direction with respect to the acceleration a, resulting in generation of the deflection due to the inertial force f at the flexible plate 32 constructed between the weight 30 and the support 31. However, since the sensor element A has the piezoelectric elements 34 disposed so as to avoid the boundary portions between the flexible parts 32 b and the nonflexible parts 32 b, in which deflection stress changes rapidly, the electric charges are stably generated in the piezoelectric elements. 34.

In addition, the sensor element A has the piezoelectric elements 34 disposed so as to avoid the boundary portions to which a larger bending stress is applied compared with the other parts. Accordingly, even if the acceleration a is extremely high, peeling-off of the piezoelectric elements 34 from the flexible plate 32 and damage to the piezoelectric body 33 hardly occur, resulting in improved sensor reliability.

On the other hand, a sensor in which piezoelectric elements 14 are provided so as to be constructed between flexible parts 12 b and nonflexible parts 12 a, as shown in FIGS. 9(a) and 9(b) (hereinafter referred to as “sensor element B”), has the piezoelectric elements disposed at parts to which the deflection stress is concentrated. Accordingly, although the sensor sensitivity is high, the deformation of the flexible plate 12 is likely to occur due to sintering shrinkage because the flexible plate cannot absorb the stress during, sintering of the piezoelectric body, and as a result, the relative position with respect to the weight and the support is likely to change.

In addition, since the sensor element B has the piezoelectric elements 14 disposed at the boundary portions, if the acceleration a is excessive, it is apprehended that peeling-off of the piezoelectric elements 14 and damage to the piezoelectric body 13 may occur due to a large impact, resulting in degradation of sensor reliability.

Differing from the sensor element A, since the electric charges generated at the piezoelectric elements 14 rapidly increase or decrease depending on slight deviations of the arrangement positions of the piezoelectric elements 14 or variations in the form of the weight 10 or the support 11 as shown in FIG. 10, there is a concern that not only may the output from each axis vary, but also the sensor accuracy may be degraded.

A sensor element having the above construction is preferably one in which the support, the weight, and the flexible plate are integrally sintered by a green-sheet lamination technique.

The green sheet is made by drying a mixed slurry of ceramic fine powder, solvent, etc., into a thin plate having characteristics such as substantial flexibility and processability, and it can be cut, punched, or adhered. Accordingly, by laminating a plurality of green sheets, compression bonding the sheets together, and then sintering the sheets, a desired shape can be formed (hereinafter the method is referred to as a “green-sheet lamination technique”).

The green-sheet lamination technique can be applied to the sensor element according to the present invention to integrally sinter the support, the weight, and the flexible plate, and the sensor element formed as mentioned above has remarkable effects that will be described as follows.

First, since the green sheet itself is easily formed as a thin plate and has a high degree of uniformity in thickness among sheets and within sheets, it is easy to make the flexible plate thin and to accurately control the thickness thereof. That is, the sensor element formed by the aforesaid method has a high sensitivity because the flexible plate thereof deflects greatly even at low accelerations, and each sensor element is of uniform characteristics and has high accuracy in the direction and the rate of acceleration obtained by the resultant force of each axis because the variation in the deflection of each flexible plate or portion of the flexible plate is small.

Second, since the thickness can be adjusted by choosing the number of thin plates to be laminated together, the thickness of the weight can be easily finely adjusted depending on the number of thin plates to be laminated together. This, in turn particularly affects the sensitivity in the X- and Y-axis directions, thereby balancing the sensitivity in the Z-axis direction. That is, the sensor element formed by the aforesaid method has a good balance of sensitivity between the X- and Y-axis directions and the Z-axis direction, and as a result, electrical calibration of the circuit can be reduced.

Third, since a sheet having a different material or thickness can be selected for each position and material on which the thin plates cut to the configuration in cross section are laminated with respect to each of the support, the weight, and the flexible plate, the sheets can be distinguished in a manner such that a highly flexible thin sheet is used for the flexible plate and a highly rigid thick sheet is used for the other materials.

In other words, since the sensor element formed by the aforesaid method is integrally formed while the flexible parts are easily deflected and the support and the weight have high rigidity, it is highly sensitive and highly accurate.

In addition to the above effects, according to the green-sheet lamination technique, since a number of sensors can be manufactured from one sheet by molding and cutting to have high productivity, an extremely economical sensor can be provided. These effects cannot be obtained by other integral-forming methods.

By using a technique of the thick-film method, such as screen-printing, for the sensor formed by the green-sheet lamination technique to form the lower electrode, the piezoelectric body and the upper electrode, a small, highly sensitive and highly accurate sensor element can be easily manufactured.

In addition, the sensor element according to the present invention is preferably an acceleration sensor element in which the support, the weight, and the flexible plate are integrally formed, and the crystal structure or crystal-grain diameter of at least one of the support, the weight, and the flexible plate preferably differs from that of the others. It is because the characteristics of each member can be controlled while taking advantage of integral forming that the numbers of the parts and processes are reduced so as to improve productivity. For example, by having the easy-bending flexible crystal structure or crystal-grain diameter for the flexible plate and by having the highly rigid crystal structure or crystal-grain diameter for the weight and the support, a highly sensitive and highly accurate sensor can be provided.

The method for controlling the characteristics of the flexible plate, etc., by the crystal structure or the crystal-grain diameter is useful for integral forming using metal as well as for ceramics. In the case of synthetic resin, the characteristics of the flexible plate, etc., may be controlled by the degree of polymerization.

In the sensor element according to the present invention, as shown in FIGS. 11(a) and 11(b), a hole 35 penetrating perpendicularly in the center of the weight 30 is preferably provided so that the space above the flexible plate 32 and a support hollow part 36 may communicate with each other.

In the case where the piezoelectric bodies are formed on the integral sintered body of the support 31, the weight 30, and the flexible plate 32 by the thick-film method and then sintered, the flexible plate 32 changes corresponding to the sintering shrinkage of the piezoelectric bodies, so that a residual stress is generated in the flexible plate 32. Since the residual stress increases the elasticity constant of the flexible plate 32, the flexible plate 32 is hardened to be difficult to deflect, resulting in degradation of the sensor sensitivity.

By providing the through hole 35 in the weight 30, the residual stress at the flexible parts is released to the weight, resulting in a state where the stress is exerted on the weight, thereby increasing the rigidity of the weight.

In addition, conventionally, as shown in FIG. 18, although when the piezoelectric elements 34 are formed on the integral sintered body 48, the relative position with respect to the flexible plate 32 and the weight 30 cannot be accurately controlled, and size accuracy is not always high, a highly accurate sensor having a high size-accuracy can be easily formed by forming the electrodes, the piezoelectric body, etc., with reference to the through hole 35.

In the sensor element according to the present invention, as shown in FIGS. 12(a) and 12(b) for example, narrow grooves 39 communicating with at least the space at the side of an outer periphery of the support 31 are preferably formed at the positions on a support end surface 31 a, which are symmetric with respect to straight lines 1 ₁ and 1 ₂ passing through the support center O on the plane including the support lower-end surface 31 a. This is because the residual stress of the flexible plate 32 can be reduced similarly to that by the through hole 35 by forming the narrow grooves 39 on the support lower-end surface 31 a after forming the piezoelectric elements, thereby reducing the degradation of the sensor sensitivity.

In addition, the narrow grooves 39 have an anchor effect of improving the adhesion strength when the sensor element is adhered and fixed to a circuit board or a package.

That is, when the sensor element is adhered and fixed to the circuit board, etc., the sensor element is preferably positioned in a state where no adhesive agent 37 exists on the support lower-end surface 31 a and a surface 38 of the circuit board, etc., and then is adhered in order to facilitate the fine adjustment of the adhering position.

By providing the narrow grooves 39 communicating with the space at the side of the outer periphery of the support 31, even if the adhesive agent 37 is adhered from the part around the sensor element after positioning the sensor element, tight adhesion can be possible because the adhesive agent 37 enters the support lower-end surface 31 a through the narrow grooves 39. Such effects are particularly effective for applications in which strong adhesive properties are required to withstand a strong impact, such as for activating an automobile air bag, etc.

Furthermore, in the sensor element according to the present invention, as shown in FIGS. 13(a) and 13(b), the weight 30 is preferably suspended so that the plane 38 including the support lower-end surface does not contact a lower end surface 30 a of the weight.

Conventionally, as shown in FIGS. 11(a) and 11(b), the sensor element is mounted in a manner such that the support 31 is loose depending on the adhesion state of the adhesive agent 37 in order to prevent the weight lower-end surface 30 a and the circuit board 38, etc., from contacting each other by the vibration of the weight 30. According to the construction of the present invention, the weight lower-end surface 30 a can be held in a state so as not to contact the circuit board 38, etc., by the construction of the sensor element itself.

The sensor element according to the present invention can be preferably manufactured by a manufacturing method including: a first step of laminating and compression bonding the green sheet of the configuration in cross section of the weight, the support, and the flexible plate to form a laminated layer; a second step of integrally sintering the laminated layer to form a sintered body: and a third step of forming piezoelectric elements on the sintered body by the thick-film method, and then sintering them.

By using the green-sheet lamination technique, the flexible plate can be made thin and homogeneous, the thickness of the weight is finely adjusted, materials for each member are optimized, and the production efficiency is improved, and also the materials for each sheet can be changed, so that materials having different crystal structure or materials having different crystal-grain diameter can be integrally formed.

Furthermore, in the manufacturing method according to the present invention, the through holes are preferably punched in the center of the green sheet of the configuration in cross section of the weight and the flexible plate and at least two corners of the green sheet of the configuration in cross section of the support and the flexible plate, and laminating and compression-bonding are preferably conducted in a state where the through holes are penetrated by the support pins to make the laminated layer.

By the construction as described above, the sensor element having the through hole at the center of the weight can be manufactured remarkably easily. In addition, using the support pins increases relative positioning accuracy of the weight, the support, and the flexible plate, and as a result, a sensor element having a high degree of size-accuracy can be manufactured.

Furthermore, as shown in FIG. 14, it is more preferable that through holes 45 and 43 be punched in the center of green sheets 40 and 42 of the configuration in cross section of the weight and the flexible plate and in the four corners of green sheets 41 and 42 of the configuration in cross section of the support and the flexible plate, and laminating and compression-bonding are performed in a state where support pins 44 pass through the through holes 45 and 43 to form a laminated layer 46 because the positioning accuracy is raised.

In addition, as shown in FIG. 15, it is preferable that after the green sheets 40, 41, and 42 of the configuration in cross section of the weight, the support, and the flexible plate have been laminated and compression bonded on a planar green sheet 47 to form the laminated layer 46, and then which has been integrally sintered to form a sintered body 48, the sintered planar green sheet 47 is cut in order that the lower end of the weight 30 can be moved.

According to the above method, since sintering is performed in a state where the support 31 lower-end is closed and the rigidity of the laminated layer 46 is maintained, there is an advantage in that deformation due to the sintering is small.

In addition, although the green sheet may be cut after sintering the laminated layer and before forming the piezoelectric elements, there is a concern that the relative position with respect to the weight and the support may change due to the sintering shrinkage of the piezoelectric body, resulting in raising the sensitivity of another axis, so that it is preferably cut after forming the piezoelectric elements.

The method of cutting the green sheet 47 is not particularly limited so long as it is cut so that the lower end of the weight 30 can be moved, but a method of cutting along the shape of the support hollow part 36 as well as a method of linearly cutting by dicing as shown in FIG. 19 is possible.

In the case of adopting the cutting method by dicing, it is possible to form at the support lower-end surface 31 a the narrow grooves 39 communicating with the space at the side of the outer periphery of the support 31 simultaneously with the cutting of the weight 30. In addition, the green sheet 47 is preferably provided with a small hole 47 a for degreasing during sintering.

Furthermore, as shown in FIG. 17, it is possible that after the planar green sheet 47 has been placed on the green sheet 41 of the configuration in cross section of the support, and furthermore the green sheets 40, 41, and 42 of the configuration in cross section of the weight, the support, and the flexible plate have been laminated and compression bonded to form the laminated layer 46, and then the laminated layer 46 has been integrally sintered to form the sintered body 48, the sintered planar green sheet 47 is cut so that the lower end of the weight can be moved.

According to the above method, as shown in FIGS. 20(a) and 20(b), it is possible to form a sensor element in which a jig such as a core, etc., is suspended so that the plane 38 including the support lower-end surface does not contact the weight lower-end surface 30 a without inserting it for each sensor element.

In addition, in this instance as well, the aforesaid effects of restraining the sintering deformation can be obviously obtained, and it is possible to form at the support lower-end surface 31 a the narrow grooves 39 communicating with the space at the side of the outer periphery of the support 31.

As shown in FIGS. 21 and 22, an acceleration sensor element 50 according to the present invention is fixed to a circuit board 51 and is sealed by a cover 53 to form an acceleration sensor 54, and when manufacturing the acceleration sensor 54, it is preferably sealed by the cover 53 in a dry atmosphere having a dew point of −40° C. or less. Consequently, condensation in the sensor can be prevented, and weight change due to the condensation at the flexible plate 32 or the weight 30 affecting the acceleration can be prevented, thereby maintaining sensor accuracy.

In addition, according to the manufacturing method of the acceleration sensor described above, in the sensor element having the through hole 35 provided at the center of the weight 30 as shown in FIGS. 11(a) and 11(b), the space above the flexible plate 32 and the support hollow part 36 communicate with each other by the through hole 35, so that a space in the support hollow part 36 is also replaced by a dry gas having a low dew point. Accordingly, the sensor element in which the weight is suspended so that the lower-end surface 30 a thereof does not contact the circuit board 38, etc., as shown in FIGS. 13(a) and 13(b) is preferable from the viewpoint of facilitating communication of the dry gas at the weight lower-end surface 30 a side.

A preferred example of the sensor element according to the present invention will be described hereinbelow with manufacturing methods therefor.

EXAMPLE 1

First, as shown in FIG. 17, the flat green sheet 47 was placed over the green sheet 41 of the configuration in cross section of the support, and furthermore the green sheets 40, 41, and 42 of the configuration in cross section of the cylindrical weight, the rectangular prism support having a cylindrical hollow part, and the flexible plate were laminated and compression bonded to form a laminated layer.

In this instance, through holes 45 and 43 were punched in the center of the green sheets 40 and 42 of the configuration in cross section of the weight and the flexible plate and in four corners of the green sheets 41 and 42 of the configuration in cross section of the support and the flexible plate, and laminating was performed in a state where support pins 44 pass through the through holes 45 and 43. Next, the laminated layer was integrally sintered at 1400 to 1500° C. for two hours to form a sintered body.

Furthermore, piezoelectric elements 34 were formed on the sintered body 48 similarly to the case in FIG. 18. In this instance, after a lower electrode 58, a piezoelectric body 33, and upper electrodes 57 a and 57 b had been sequentially disposed by screen printing only at the flexible parts. 32 a of the flexible plate similarly to the sensor in FIGS. 1(a) and 1(b), sintering was performed at 1200 to 1300° C. for three hours.

In addition, as for the piezoelectric elements, after printing the lower electrode 58 and the piezoelectric body 33 to have a doughnut shape similar to the sensor in FIGS. 1(a) and 1(b), they are divided into eight parts, and then the upper electrodes 57 a and 57 b were printed, thereby forming a total of eight piezoelectric elements. Finally, similarly to the case in FIG. 19, the planar green sheet is diced by the die, and the diced green sheet contacts the cylindrical weight 30, and then the lower end of the weight 30 was cut off, thereby yielding the sensor element of the invention.

The sensor element was made into an acceleration sensor by the following method. That is, as shown in FIGS. 21 and 22, after a sensor element 50 had been positioned at a desired position on a circuit board 51, an adhesive agent was injected from the outer periphery of the support lower-end surface, thereby fixing the sensor element 50 on the circuit board 51.

An acceleration sensor 54 was manufactured in a manner such that after the circuit board 51 had been fixed to a CAN package base 52 by soldering, and in which a dry nitrogen gas had been used to purge to produce a dry atmosphere having a dew point of −40° C. or less in a CAN seal device having a glove box (Japan Avionics), a CAN package case 53 was airtightly welded for sealing.

INDUSTRIAL APPLICABILITY

According to the sensor element of the present invention, as described above, deformation of the flexible plate due to the sintering shrinkage is unlikely to occur, and the piezoelectric elements are disposed so as to avoid the boundary portions between the flexible parts and the nonflexible parts at which a deflection stress rapidly changes. Accordingly, since the electric charges are stably generated in the piezoelectric elements, and also the electric charges generated in the piezoelectric elements do not rapidly increase or decrease depending on slight deviations in the arrangement positions of the piezoelectric elements or form variations in the weight or the support, the output variation from each axis can be restrained, resulting in improved sensor accuracy.

In addition, even if the sensor element receives a rapid acceleration, the piezoelectric elements will not peel off from the flexible plate, and the piezoelectric body is unlikely to be damaged, resulting in improved sensor reliability.

In addition, according to the manufacturing method of the present invention, a highly sensitive, highly accurate, and highly reliable sensor element can be easily manufactured. 

What is claimed is:
 1. An acceleration sensor element comprising: a weight; a support having a lower end surface and a hollow part, said support being disposed around the weight as a center; a flexible plate disposed horizontally across the support so as to suspend the weight at the center of the hollow part of said support; and at least one piezoelectric element disposed on the flexible plate, and comprising a piezoelectric body sandwiched by at least one set of electrodes; wherein the support, the weight, and the flexible plate are integrally sintered by a green-sheet lamination technique; wherein acceleration applied from the exterior is converted to a deflection of said flexible plate based on the behavior of said weight corresponding to the acceleration, and the direction and the rate of said acceleration are three-dimensionally determined by electric charges generated in said piezoelectric body corresponding to the deflection of the flexible plate; wherein the piezoelectric elements are disposed only at flexible parts of said flexible plate; and wherein grooves communicating with at least a space at the side of an outer periphery of the support are formed at positions on the support lower end surface, said grooves being symmetric with respect to straight lines passing through a center of the support and parallel to a plane including the support lower end surface.
 2. An acceleration sensor element as in claim 1, wherein a crystal structure of at least one of the support, the weight, and the flexible plate is different from the others.
 3. An acceleration sensor element as in claim 1, wherein a crystal-grain diameter of at least one of the support, the weight, and the flexible plate is different from the others.
 4. An acceleration sensor element as in claim 1, further comprising a hole penetrating perpendicularly through the center of the weight so that a space above the flexible plate and the hollow part of the support communicate with each other.
 5. An acceleration sensor element as in claim 1, wherein the weight is suspended so that a plane including the lower end surface of the support does not contact a lower end surface of the weight. 